3d printed micro channel plate, method of making and using 3d printed micro channel plate

ABSTRACT

The invention provides a gain device having a plurality of channels having a polygonal shape with four or more sides. The invention also provides a method for producing microchannel plates (MCPs) having the steps of providing a pre-polymer; and directing a laser over the pre-polymer into a pre-determined pattern. Also provided is method for efficiently 3D printing an object.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority as a divisional of U.S.patent application Ser. No. 15/718,407, filed on Sep. 28, 2017,currently pending.

CONTRACTUAL ORIGIN OF THE INVENTION

The U.S. Government has rights in this invention pursuant to ContractNo. DE-AC02-06CH11357 between the U.S. Department of Energy and UChicagoArgonne, LLC, representing Argonne National Laboratory.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to micro capillary arrays and more specifically,this invention relates to a method of 3D printing microchannel platesfor various uses, where 3D printing greatly decreases the time needed toproduce the arrays and reduces the cost of same versus state of the artmicrochannel plates.

2. Background of the Invention

Microchannel Plates (MCPs) are plates defining regular, parallel arraysof microscopic channels. These channels are normally cylindrical andpass through the entire thickness of the plate. Standard MCPs are madefrom glass.

The classic use of an MCP is as an electron multiplier. In use as anelectron multiplier a voltage is applied along the length of eachchannel. Each channel is coated with a suitable tunable resistive layerand an electron emissive layer. With this configuration, an electronthat collides with the wall of a channel will produce several moreelectrons which will then produce several more electrons when thoseelectrons collide with the wall of a channel. In this way, MCPs can beused in the same way as classic electron multiplier devices.Furthermore, the MCP can amplify a pattern of electrons incident on thefront surface because each pore acts independently. In a classicelectron multiplier configuration, MCPs are useful in devices designedto multiply incident energy such as night vision devices. MCPs mayfurther be used as a sieve to separate ³He and ⁴He, filters to trapviruses in the air, as a template for the parallel synthesis ofmicrotubes or microwires, and membranes for water purification.

With modification, standard MCPs can serve as neutron detectors. In sucha configuration, the material comprising the channels is modified torelease multipliable and detectable moieties upon incidence of aneutron. For example, traditional glass MCPs can be doped with ¹⁰B that,upon incidence with a neutron, release a ⁷Li particle, an alphaparticle, and gamma radiation. When the alpha reaches a channel of anMCP and produces one or more electrons, the channels act as classicelectron multipliers so that each electron produced after a neutroncollision is multiplied into many electrons and a detectable number ofelectrons reach a detector at the bottom of the channels.

State of the art MCPs, as referenced above, are made from glass asdescribed in U.S. Pat. No. 9,082,907. The process for making prior artMCPs is a complicated series of steps involving glass melting, molding,extruding, etching, and packing individually created channels into aform. Such a process is a time and effort intensive process requiringthe input of experienced artisans. As such, MCPs made by this methodcost between ˜$50-$200/cm². These costs are prohibitive in many cases,leaving the need for a more efficient, less labor intensive way to makeMCPs.

3D printing is rapidly advancing and is presently becoming such a methodfor creating high-precision devices on-demand and without the need forhighly skilled artisans. For these objectives, 3D printing is aground-breaking tool that is readily changing the way consumer andhigh-precision scientific objects are made. However, 3D printing ofsmall-scale, precision objects requires much more time than printinglarger devices. For instance, it would take approximately 72 hours toprint a 1 cm² MCP that is 0.15 mm thick. Moreover, the printing mediumfor small-scale, precision optics is a polymer that does not possess thenecessary resistive and electron emissive properties for an MCP.

A need in the art exists for cheaper and more widely available MCPs.There is also a need for an economical method for producing such MCPs.The MCPs should be equally applicable to current and future scientificneeds as state-of-the-art glass MCPs. And, the method for making thesenew MCPs should be cheap, fast, and reliable.

SUMMARY OF INVENTION

An object of the invention is to provide a method for creating MCPs thatovercomes many of the disadvantages of the prior art.

Another object of the invention is to provide a method for creating MCPsthat is automatic and reliable. A feature of the invention is the use of3D printing to create MCPs. An advantage of the invention is that, oncethe system to carry out the 3D printing is in place, there is no need toemploy a skilled artisan to assemble the invented MCPs. Another featureof the invention is that the invented 3D printing method produces MCPsthat are at least one cm in diameter and 1.2 mm thick in approximately24 hours. A combination of a rapid printing method overseen bynon-skilled personnel results in cost savings and reduced prices of MCP,compared to traditional MCPs. For example, MCPs created using theinstant method cost on the order of $1/cm² compared to approximately$50-$200/cm² for prior art MCPs.

Another object of the invention is to provide a method for reducing thetime needed to 3D print the invented MCP. A feature of the invention isthat an out-of-the box 3D printer is used in making the invented MCPthat is then modified to operate more efficiently through software anddesigned print orders. An advantage of the invention is that the toolsneeded to use and the instant invention are widely available.

Still another object of the invention is to provide precision MCPs withsuperior qualities to state-of-the art, glass MCPs. A feature of theinvention is that the created MCPs outperform glass MCPs in severalrespects, including open area ratio and gain. An advantage of theinvention is that the produced MCPs feature open area ratios of at least97% and gain of at least 10⁴ for 1.2 mm thick MCPs, where the gain valueincreases with MCP thickness.

Yet another object of the invention is to use the invented MCPs invarious devices requiring electron multiplication. A feature of theinvention is that the invented MCPs can be used to multiply incomingelectrons in order to detect or increase detection of various types ofincident particles or energy. An advantage of the invention is that theinvented MCPs are applicable to many technologies where electronmultiplication is used to detect incoming radiation or particles.

Still yet another object of the invention is to use the invented MCPs asa neutron detector. A feature of the invention is the use of ¹⁰B withinthe 3D printer “ink” such that the printed MCPs eject and multiplyelectrons upon the incidence of neutrons. An advantage of the inventionis that the use of inexpensive 3D printing methods with ¹⁰B doped “inks”results in cheap neutron detectors that are very small and can be usedin the field to detect the presence of fissile materials such as uraniumor plutonium.

Yet another aspect of the invention is to use atomic layer deposition(ALD) and sequential infiltration synthesis (SIS) to apply thin films onall internal and external surfaces of the 3D printed MCPs to impartresistive and secondary emissive properties. Unlike conventionalcapillary arrays composed of dense glass, the 3D printed polymer hasintrinsic porosity due to the free volume characteristic of organicpolymers. A feature of the invention is using SIS to infiltrate and sealpores in the near-surface region of printed MCPs pores so that thesubsequent ALD proceeds in a controlled fashion.

Briefly, the invention provides 1 gain device comprising a plurality ofchannels having a polygonal shape with four or more sides.

Also provided is a method for producing microchannel plates (MCPs)comprising: providing a pre-polymer; and directing a laser over thepre-polymer into a pre-determined pattern.

Still further provided is A method for efficiently 3D printing an objectcomprising: a) providing a pre-polymer into a sample holder; b)directing a laser over the pre-polymer in a predetermined pattern tocreate a layer of an object having a height, H; c) raising the sampleholder along a latitudinal axis that is parallel to the height of theobject and runs through the center of the layer by a distance equal toH; d) directing the laser over the pre-polymer in the predeterminedpattern; e) repeating steps a)-d) until the object has a firstpredetermined area and a predetermined second height; f) resetting theposition of the sample holder; g) moving the sample holder along alatitudinal axis of the layer a predetermined distance; h) repeatingsteps a)-g) until the object has a final area; i) resetting the positionof the sample holder; j) raising the sample holder along a latitudinalaxis through the center of the layer by a distance of the predeterminedsecond height plus H; and k) repeating steps a)-j) until the object hasa final area and final height.

BRIEF DESCRIPTION OF DRAWING

The invention together with the above and other objects and advantageswill be best understood from the following detailed description of thepreferred embodiment of the invention shown in the accompanyingdrawings, wherein:

FIGS. 1A and 1B are schematic diagrams showing the production of priorart MCPs;

FIG. 2 depicts the 3D printer used in the instant invention, inaccordance with the features of the present invention;

FIGS. 3A-3B show a schematic of a 3D printing protocol to print a MCPwith hexagonal channels, in accordance with the features of the presentinvention;

FIG. 3C is a plan view of the path taken by the laser of 3D printer inprinting the instant Microchannel Plates, in accordance with thefeatures of the present invention;

FIG. 4A depicts the chemical components of the photoresist used to printthe invented MCPs, in accordance with the features of the presentinvention;

FIG. 4B depicts a chemical reaction between boric acid and a photoresistmonomer in accordance with features of the present invention;

FIGS. 4C-E depict various chemical moieties that can be used to dopephotoresist with ¹⁰B isotope, in accordance with the features of thepresent invention

FIG. 5A is an SEM image of a printed MCP, in accordance with thefeatures of the present invention;

FIG. 5B is a schematic, perspective view of a single channel from a MCP,in accordance with the features of the present invention;

FIG. 5C depicts an SEM image showing the detail of the ends of a printedMCP, in accordance with the features of the instant invention;

FIG. 6 is a photograph of a finished MCP prior to coating, in accordancewith the features of the present invention;

FIG. 7A depicts a cross-section of a functionalized MCP, in accordancewith the features of the present invention;

FIG. 7B depicts an SEM image of a MCP channel coated using SIS and ALD,in accordance with the features of the present invention;

FIG. 7C depicts a protocol for using sequential infiltration synthesisto infiltrate pores of the MCPs, in accordance with the features of thepresent invention;

FIG. 8 is a simplified cross-section of a photodetector using theinvented MCPs, in accordance with the features of the present invention;

FIG. 9 is an exploded view of the components used in a photodetectorusing the instant MCPs, in accordance with the features of the presentinvention;

FIG. 10 is a photograph of an assembled photodetector that uses theinstant MCPs, in accordance with the features of the present invention;

FIG. 11 is a simplified cross-section of a neutron detector using theinstant MCPs, in accordance with the features of the present invention;

FIG. 12 depicts a phosphor screen image showing collision with anelectron cloud that has been multiplied using an MCP, in accordance withthe features of the present invention; and

FIG. 13 depicts a plot of relative gain of a 3D printed MCP versus biasvoltage applied to the MCP, according to the features of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing summary, as well as the following detailed description ofcertain embodiments of the present invention, will be better understoodwhen read in conjunction with the appended drawings.

All numeric values are herein assumed to be modified by the term“about”, whether or not explicitly indicated. The term “about” generallyrefers to a range of numbers that one of skill in the art would considerequivalent to the recited value (e.g., having the same function orresult). In many instances, the terms “about” may include numbers thatare rounded to the nearest significant figure.

The recitation of numerical ranges by endpoints includes all numberswithin that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and5).

The following detailed description should be read with reference to thedrawings in which similar elements in different drawings are numberedthe same. The drawings, which are not necessarily to scale, depictillustrative embodiments and are not intended to limit the scope of theinvention.

As used herein, an element or step recited in the singular and precededwith the word “a” or “an” should be understood as not excluding pluralsaid elements or steps, unless such exclusion is explicitly stated. Asused in this specification and the appended claims, the term “or” isgenerally employed in its sense including “and/or” unless the contentclearly dictates otherwise.

Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising” or “having”an element or a plurality of elements having a particular property mayinclude additional such elements not having that property.

The invention provides a method for 3D printing micro channel plates(MCPs) and devices using the printed MCPs.

FIG. 1A is a schematic diagram showing a state of the art method forproducing MCPs. A plurality of glass rods is created by nesting twotypes of glass 11 within a Low-Z, bulk, casing 10, and drawing the rodsto be a first size rod 12. Those rods are then stacked and extruded tocreate smaller rods 14. Then, in a series of final steps, bundles ofsmaller rods 14 are then heated, sliced, etched, polished, and hydrogenfired to create final MCPs 16. The hydrogen firing step imparts both theresistive and emissive properties to the MCP surface.

FIG. 1B is a schematic diagram showing another state of the art methodfor producing MCPs. A plurality of hollow glass capillaries is createdby drawing hollow glass tubes 11 to be a first size 12. Thosecapillaries are then stacked and extruded to create smaller capillaries14. Then, in a series of final steps, bundles of smaller capillaries 14are then heated, sliced, polished, and ALD coated to create final MCPs16. The resistive property of the MCP is imparted by the ALD resistivecoating in step 10, and the emissive property of the MCP is imparted bythe ALD emissive coating in step 11. Several advantages of the MCPfabrication method shown in FIG. 1B over that in FIG. 1A are theelimination of assembling and etching steps, and the ability toindependently tune the resistive and emissive properties of the MCP.

The series of steps shown in FIGS. 1A-1B are labor intensive, precise,and require a skilled artisan. As such, the process is time and laborintensive. The final MCPs created using the prior art method, therefore,cost between $50-$200 per cm².

3D Printing Detail

In place of the more difficult and time intensive state-of-the-artefforts of manufacturing MCPs, the inventors have utilized 3D printingto manufacture MCPs more quickly, with less expense, and with superiorproperties. Specifically, the inventors have utilized two photonpolymerization of IR-curable photoresist using a single-head 3D printer.Preferably, the printer has approximately sub 0.5 μm 2D lateralresolution and approximately sub 1 μm vertical resolution. Any 3Dprinter capable of these parameters is suitable for use in the instantinvention. Commercially available printers with these capabilities arewidespread and include the micron resolution printer (PhotonicProfessional) from Nanoscribe GmbH, of Eggenstein-Leopoldshafen,Germany.

FIG. 2 depicts the 3D printer setup 20 as used in the instant invention.The printer setup 20 is comprised of a computer 22 that runs the printer20. The computer is operated by a user interacting with a terminal thatruns software and a user interface 24 needed to instruct the printer 20.The exemplary Nanoscribe printer comes with two programs written by thecompany to run the printer, Nanowrite and a compiled version ofNanowrite called Describe.

The printer 20 itself is comprised of a laser cabinet 26 (containing alaser and its necessary operating optics), a microscope 28 for providinga high resolution field of view for focusing the printer's laser, ascanning unit 30, and a positioning system 32. The scanning unit 30comprises pivotable galvo mirrors for quick, short distance lasermovements. In an embodiment, the galvo mirrors (or plates) repositionthe printing laser beam very quickly (10⁶ μm per second) and preciselyin the x-y plane (parallel to the ground) over a range of approximately250 μm. The positioning system comprises three piezoelectric stages, oneto move the sample holder 34 in each of the x, y, and z directions, anda mechanical stage capable of moving the sample holder 34 in anydirection (x, y, or z). The piezoelectric stages are slower than thegalvo mirrors (2·10² μm per second) but operate over a larger distance,approximately 300 μm, but move with high precision. The mechanical stageis the slowest (2 μm per second) and least precise of the threesample/laser moving apparatuses but offers the largest range ofmovement, 100 mm in any direction. The printer uses a continuous pulse,femtosecond laser. The Ti:Sapphire laser has a wavelength ofapproximately 780 nm. In an embodiment, the printer is a single headprinter operating under a two photon paradigm. Alternatively, theinvention can utilize any printer with sufficient resolution to make theMCPs described herein.

The speed and range of motion given for the movement apparatusesdiscussed above are exemplary and not meant to be limiting. Customizedprinters may be used in the instant invention. In these custom printersthe precision of motion of the movement apparatuses are repeatable tothe spatial resolution of the object being printed. For example, usingthe printer described above, the precision is measured to approximately50 nm for the galvo plates and piezoelectric stage, the mechanical stageprecise to approximately 100 nm. In a custom printer, precision may beimproved to 50 nm on all movement apparatuses.

The printer used in the instant invention is a single head printer thatoperates under the 2γ (two photon) paradigm. Under the two photonprinciple of operation, a user provides an amorphous and non-shapedpre-polymer called photoresist to the sample holder 34 of the printer.The laser is then activated and is directed to the photoresist via theprinter's software and positioning systems (galvo mirrors, piezoelectricand mechanical sample stages) in a pre-programmed series of movements tomake a shape/configuration designed by a user. When a voxel of thepre-polymer absorbs two photons of IR light generated by the printer'slaser, the photoresist polymerizes and hardens. (A voxel represents thesmallest unit of pre-polymer.) Thus, in order to create a desired shapeusing the default print settings, a user programs that shape into theprinter 20 setup using a user interface. The programed shape is thenmade layer by layer, voxel (smallest volume of pre-polymer polymerizedby two photons from the laser) by voxel, until the pre-determined shapeis completed.

An important feature of the invention is the improved 3D printingprotocol invented by the inventors. Using a 3D printer's standardsettings, the printer generates the desired object one layer at a timewithout concern for the distance between two subsequent sweeps of thelaser or the amount of a layer that is gone over more than once by thelaser. In an embodiment, the inventors have discovered a 3D printingpath that leverages the relative range, precision, and speed of thethree laser positioning apparatuses (galvo mirrors, piezoelectricstages, and mechanical stage) to maximize the speed of a desired 3Dprinting. The increase in speed is generated by eliminating essentiallyall non-printing motions, moving the sample as quickly and precisely aspossible, and repeating minimal sweeps of the laser per layer.

FIGS. 3A-3B show a schematic of the invented 3D printing protocol. Theprotocol begins by using the galvo plates to move the laser over thephoto-resist prepolymer in a predetermined series of movements (aportion of the series of movements to draw the invented MCPs shown inFIG. 3C). This first step (step IA) continues building a single layerone voxel high until the range of motion of the galvo plates isexhausted, giving a first layer 120 having an area of approximately450×450 μm². The z-direction piezoelectric stage then moves the sampleholder up in the z direction by the distance of one layer (approximately100-1000 nm), and either the x or y piezoelectric stage moves slightlyin the x or y direction 40 nm to offset subsequent layers by a biasangle as discussed below (step IB). The galvo plates then move theprinter's laser through the same predetermined pattern to produce asecond layer 122 on top of the first (step IC). FIG. 3A shows the result124 of the first iteration of step IB and IC. This first step isrepeated a plurality of times until the developing structure has a sidemeasuring at least 250 μm (450×450 μm² in this example) and thestructure has a height of 300 μm (step ID). In practice, the firstiteration of step ID is complete for the described hexagonal MCPs whenthe generally hexagonally shaped structure 124 has an edge length ofapproximately 450 μm and height of approximately 300 μm.

In the second step of the 3D printing protocol, the piezoelectric stagesare all reset and the mechanical stage moves to an adjacent area to thestructure already printed (Step II). Step 1 (IA-D) is then re-performedin this area until there is an additional portion of the developingstructure that is 250 μm on one side with a height of 300 μm. FIG. 3Ashows the result of the first iteration of step II and first repetitionof steps IA-D 126.

In step 3, steps IA-D and 2 are repeated until the structure printed hasthe desired area and a thickness of 300 μm 128 (step III). In step 4,the position of the sample holder is reset and subsequently raised 300nm in the z direction by the mechanical stage (step IV). Subsequently,steps IA-IV are repeated until the printed plate 130 reaches the desiredthickness (step V). The dimensions given here are exemplary andreference using the above-described 3D printer to print the inventedMCPs. A person having ordinary skill in the art can easily adapt theabove protocol to build any structure of any dimension given a suitableprinter.

Other than the efforts to generally improve the speed and scale ofobjects that can be printed using the instant method, the inventors havediscovered the optimal laser path for producing a layer of hexagonalchannels 52 discussed below and shown in FIG. 3C. The optimal laser pathis represented with the schematic in FIG. 3C, the laser movementsnumbered to show the sequential laser path used to create a layer of theinstant MCPs. As can be seen in FIG. 3C, the optimal laser path for asingle layer of the hexagonal channels draws each layer with minimalpath repetition. For example, each hexagon is drawn having only one sidegone over by the laser more than once.

Upon completion of MCP printing, the rough MCPs are dissolved fromunreacted pre-polymer using a solvent such as PGMEA. Polymerization ofthe MCPs is then completed with UV lamp irradiation. The MCPs can thenbe functionalized as described herein. In a final step, the ends (85,87, 89, and 91 as shown in FIG. 8) are polished.

Using the above-referenced method for printing MCPs, the instantinvention has enabled the printing of MCP plates as pictured in FIG. 6.MCPs that are one cm in diameter and 1.2 mm thick can be printed via theinvented method in approximately less than 24 hours, and typicallybetween 18 and 24 hours. Previously, such MCPs took approximately 800hours to produce the same plates. Compared to the cost ofstate-of-the-art glass MCPs, the instant MCPs can be produced at100-1000 fold reduction in cost depending on the size of the plate. Inan embodiment, the instant method can be used to print MCPs up to 100 cmin diameter and 3.6 mm thick in under 48 hours. The inventors envisionthat these sizes and times can be improved with the production of acustomized printer with additional print heads and improved optics, i.e.using larger microscope objectives.

The instant printing method can be scaled to produce square meter scaleplates in similar times using improved printers. These printers can useall reflecting microscope objectives whose size allows 100 times largerfields of view than the microscope objectives used in the commercialprinter mentioned above while maximizing numerical aperture. This hasthe advantage of higher resolution with the highest vertical resolution.Using multiple laser beams also dramatically increases the printingspeed. Additionally, an improved printer may use many lasers or a singlemore intense laser with an array of shutters.

Any two photon pre-polymer photoresist can be used in the instantinvention. An exemplary pre-polymer photoresist is IP Dip purchased fromNanoscribe GMBH of Eggenstein-Leopoldshafen, Germany. FIG. 4A shows thethree components of Nanoscribe's IP Dip product: Pentaerythritoltriacrylate 40 (60-80 wt %),9,9-Bis[4-(2Acryloyloxyethoxy)phenyl]fluorene 42 (<24 wt %), and2-(o-Phenylphenoxy)ethyl acrylate 44 (<24 wt %). This photoresistmixture is exemplary and not meant to be limiting. Suitable two photonpre-polymer photoresist typically comprise 50-80 wt % of a monomer thatpolymerizes with the application of light, 15-30 wt % of a catalyst tocatalyze polymerization of the monomer, and a viscosity controllingmoiety whose concentration is adjusted according to the amounts used ofthe monomer and catalyst. The inventors have discovered that thephotoresist can be modified as is discussed, infra, in order to conferdesired electrical or neutron reactive properties.

In alternative embodiments, additional moieties are added to thephotoresist prepolymer mixture in order to confer additionalfunctionality to the printed MCPs. For example, a resistive layer isadded to the MCPs as discussed below to allow for a biasing voltageacross the sides of an MCP to accelerate electrons along the length ofchannels. In an alternative embodiment, the photoresist used to printMCPs is doped with graphene oxide or any other inert, conductivematerial such that the printed MCP already has the desired resistiveproperties. Such an embodiment uses photoresist containing approximately0.1-3 wt % graphene oxide, the percentage of the other components of thephotoresist adjusted according to the amount of graphene oxide. Otherinert, conductive materials may include nanoparticles, nanotubes,nanowires, nanoflakes or other suitably shaped nanomaterials composed ofcarbon, metal, metal oxide, metal nitride, or mixtures thereof.

An important purpose of the instant MCPs is for use in neutron detectorsrelying on ¹⁰B. For use in such neutron detectors, the instant MCPs areprinted using a ¹⁰B doped photoresist such that the printed MCPs alreadyincorporate the isotope. The photoresist can be doped in a number ofways to produce printed MCPs with sufficient ¹⁰B. An exemplary method isa reaction of boric acid (¹⁰BH₃O₃) with the photoresist monomer(Pentaerythritol triacrylate) in methanol as shown in reaction 45 inFIG. 4B. The boric acid and methanol react to form trimethyl borate(¹⁰B(O—CH₃)₃) that is soluble in the polymer, and the trimethyl boratereacts with the OH group in the polymer to form a new C—O—B bond.Alternatively, the ¹⁰BH₃O₃ and methanol can be replaced with pure¹⁰B(O—CH₃)₃.

Another method is to add borated compounds to the pre-polymerizedphotoresist without reaction. Example compounds 46, 47, and 48 as shownin FIGS. 4C-E respectively use large complexes containing ¹⁰B polygonsat their center. These large compounds are preferable for doping thephotoresist with ¹⁰B compared to smaller compounds as smaller compoundssuch as triethyl boron reduce the polymerizability of the photoresist asthe concentration of ¹⁰B rises. The necessary thickness of a MCP dopedwith ¹⁰B for use in a neutron detector is proportional to the percent¹⁰B in the photoresist. For example, a 1 mm thick MCP would need to beapproximately 50 wt % of the ¹⁰B compounds shown FIGS. 4C-E for use inan MCP. In an alternative embodiment, the photoresist pre-polymer isdoped with ²⁸Si instead of or in combination with ¹⁰B. Yet anothermethod is to perform SIS using ¹⁰B(O—CH₃)₃ or another ¹⁰B precursor toinfuse the polymer matrix with an inorganic compound containing ¹⁰B.Such SIS may be followed by ALD deposition of one or more of coatingscomprising any of the ¹⁰B compounds described herein.

Micro Channel Plate Detail

The instant invention uses the precision of 3D printing to create MCPsas shown in FIG. 5A. As shown in FIG. 5A, the MCP 50 defineslongitudinally extending hexagonal channels 52, a shape not possibleusing previous MCP manufacturing methods. These hexagonal channels 52provide MCPs with higher open area ratios when compared to prior artcircular channels as shown in FIG. 1A. In an embodiment, the hexagonalchannels are substantially straight. Alternatively, the channels 52 havea bias angle θ wherein the channel is angled with respect to thelatitudinal axis a of the MCP 50, such that the angle is greater than 0degrees and less than 90 degrees. This is demonstrated in FIG. 5B whichshows a schematic of a single channel 52. Preferably, the bias angle θis between approximately 0° and approximately 60°, more preferablybetween approximately 0° and approximately 40°, and typically betweenapproximately 0° and approximately 30°. In FIG. 5B, the angle isdepicted as approximately 85 degrees relative to the latitudinal axis.The channels 52 are defined by thin walls 54 of polymerized photoresistthat are approximately 100 nm thick, thereby providing channels 52 thatare approximately 10 μm in diameter.

The exemplary embodiment shown and described herein produces MCPs withhexagonal channels. However, the hexagonal shape is exemplary and notmeant to be limiting. FIG. 3A-B's protocol can be used to produce MCPsof any shape and size having channels of any shape and size. The 3Dprinting protocol discussed and shown herein can be used to efficientlyprint any MCP with transversely extending channels of any regular orirregularly shape, or even a single MCP incorporating more than onechannel shape at a time. In an embodiment, the MCP channels have apolygonal shape with four or more sides.

Using the 3D printing process described, supra, the instant MCPs arehighly precise and superior in several respects to conventional glassMCPs. FIG. 6 shows a photograph of a finished MCP 60. In an embodiment,the produced MCPs are highly flat where produced MCPs have beenempirically measured to have flatness error of less than 1 μm. Further,the MCPs 60 have significantly higher open area ratios when compared toconventional class MCPs. In an embodiment, the produced MCPs preferablyhave open area ratios more than approximately 80%, preferably more thanapproximately 90%, and typically more than approximately 95%. Theselarge open areas facilitate a high probability that a particle incidenton the produced MCP will enter one of the channels as shown in FIG. 5Bas desired. These are highly desired properties for MCPs that are thenused as radiation multiplying portions of various devices once the MCPsare functionalized. Also, the instant MCPs provide approximately 10⁴gain with 1.2 mm thick plates, a 10-fold improvement over the prior art.Additionally, gain improves to more than 10⁷ when the MCPs are printedto 2.4 mm thick.

A salient feature of the invention is that the printed MCPs can becompletely customized. As stated above, the invented MCPs can be printedhaving walls 54 that are approximately 100 nm thick. Such thin walls andtherefore large channels and high open area ratios are preferable foruse in gain applications. Where the instant MCPs are used in neutrondetecting configurations, the channels are printed to approximately 1 μmthickness.

In an embodiment, the MCPs can define channels of any shape and biasangle. The instant MCPs are shown and described as hexagonal. However,the channels can also be square, rectangular, or circular. Additionally,the channels can be printed having any bias angle or with multiple biasangles (i.e. the bias angle changes at a point along a channel'slongitudinal axis). Similarly, the instant method can be used to printchannels that change shape along their longitudinal length, or form acorkscrew pattern. The channels can be printed to have different shapesat different spatial locations across the MCP, and the MCP can beprinted flat or curved.

Functionalization of MCP for Electron Multiplication Detail

FIGS. 5A and 6 depict MCPs that are produced in the instant inventionbefore the MCPs are modified for use. For use as a traditional electronmultiplier or neutron detector, the MCPs are first functionalized sothat the channels have the proper electronic properties. Theseproperties are conferred by applying coatings to the ends (52 a in FIGS.5B and 5C) and interior surface 54 of the channels. A coated channel 70is depicted in FIG. 7A.

Both ends 52 a of the channels 52 are coated with a conductive material72 so that the coated ends 52 a, b (wherein 52 a designates the upstreamend and 52 b designates a downstream end) serve as electrodes for theapplication of a voltage difference across the channels 52. Any materialsufficiently conductive to serve as an electrode when deposited as afilm is suitable for the conductive material to coat the ends. Exemplarymaterials include gold, platinum, palladium, copper, nichrome, andcombinations thereof. The electrode material can be deposited via anysuitable means for depositing conductive thin films that isline-of-site. This line-of-site requirement ensures that the electrodespenetrate a fixed depth of approximately one pore diameter into thepores to provide adequate electrical contact with the resistive porewall, but do not extend so far as to create an electrical short throughthe pores. Exemplary line-of-site deposition processes for the electrodematerial include thermal evaporation, electron beam evaporation, andsputtering. As shown in FIG. 5C, the ends 52 a, b of the channels makeup an unbroken grid. Thus, a voltage applied to any portion of theelectrode coating 72 on one end 52 a of the channels distributes chargeacross all of the electrode coating 72 on that one end of an assembledMCP.

The interior surface 54 of the channels 52 also receive a plurality ofcoatings to facilitate electron multiplication. Prior to applying thesecoatings, surface pores in the interior surface 54 can be sealed usingSIS. For instance, 25 cycles of SIS Al₂O₃ can be performed to accomplishthe sealing. A tunable resistive coating 74 is first applied to athickness between approximately 1 nm and approximately 1000 nm on theinterior walls of the channels 52. This coating facilitates a biasvoltage across the channels 52 when a voltage is applied to electrodeson the ends 52 a of the channels. In an embodiment, the resistivecoating is made from a combination of alumina (Al₂O₃) and tungsten, andis applied via atomic layer deposition (ALD). For instance, tunableresistive coating 74 can be a 50 nm film prepared using a W:Al₂O₃ ratioof 33% where the ALD cycles are executed as: W—Al₂O₃—Al₂O₃—W—Al₂O₃—Al₂O₃. . . to provide an MCP resistance of 2.3×10⁹ Ohms. The resistance ofthe MCP can be tuned to any desired value by adjusting the W:Al₂O₃ ratioand thickness during the growth of the tunable resistive coating 74.FIG. 7B depicts a MCP channel 52 having an ALD coating 77 on itsinterior surface. This coating is composed of a resistive coating 74 anda secondary electron emissive coating 76. Alternatively, graphene oxideor any suitable inert, conductive material can be mixed with thephotoresist polymer prior to 3D printing, and in this case the resistivecoating 74 can be eliminated. A secondary emission layer 76 (FIG. 7A)comprising Al₂O₃, MgO, or another material with a high secondaryelectron emission coefficient is then deposited to a thickness betweenapproximately 1 nm and approximately 100 nm on the resistive layer, forexample via ALD. For instance, secondary emission layer 76 can be an ALDAl₂O₃ film with a thickness of 10 nm prepared using 75 ALD Al₂O₃ cycles.In an electron multiplication configuration, it is this secondaryemission layer that releases electrons when struck by an initialelectron when there is a voltage applied across the channels 52. Afterapplication of the coatings, functionalized MCPs can be assembled into adetector apparatus as shown in FIG. 8.

The tunable resistance coating 74 and the secondary electron emissivecoating 76 can be deposited by any suitable method that results inuniform, precise and conformal coatings such as ALD.

To prevent unwanted chemical vapor deposition (CVD) during the atomiclayer deposition (ALD) coating of the resistive coating 74 and theemissive coating 76, the pores of the porous polymer surface 54 canfirst be sealed using sequential infiltration synthesis (SIS). Anexemplary SIS reaction is shown in FIG. 7C. That figure uses the polymerpentaerythritol triacrylate (40 from FIG. 4A) as an exemplary startingpolymer, but any polymer with suitable functional groups such as thecarbonyl (C═O) groups 200 will facilitate the SIS. In step 1 of the SIS,the polymer 40 is exposed to trimethyl aluminum (TMA) vapor 201, and theTMA reacts with a fraction of the carbonyl groups 200 to form a new O—Albond to the chemisorbed dimethyl aluminum (DMA) 202, and the thirdmethyl (CH₃) group 203 bonds to the carbon of the carbonyl 200. In step2, the polymer is exposed to H₂O vapor 204 which reacts with the DMA toform hydroxyl (OH) groups 205. This TMA-H₂O process can be repeatedmultiple times to develop Al₂O₃ clusters inside of the near surfaceregion of the MCP to densify the polymer and seal the pores.Consequently, the ALD resistive coating 74 and emissive coating 76 willgrow only on the outer surface of the polymer, and CVD will be avoided.

Further detail of the SIS and ALD procedures used to functionalize theinvented MCPs are presented in one of the prior patents to theinventors, U.S. Pat. No. 9,139,905, the entirety of which isincorporated by reference herein.

Photodetector Detail

FIG. 8 depicts a cross section of a photodetector 80 utilizing twofunctionalized MCPs 82 a, b. The functional elements of the detector 80are positioned between two windows 81 that allow for the passage oflight and may be glass, quartz or sapphire, a superior window 81 a andan inferior window 81 b. The superior window 81 a is positioned superiorto and in contact with a photocathode 83. A first functionalized MCP 82a is positioned inferior to the photocathode 83 and superior to andspaced away from the second functionalized MCP 82 b along thelongitudinal axis line a-a of the detector 80. As discussed above, theinstant MCPs may be produced as to have a bias angle. In thephotodetector 80, the first functionalized MCP 82 a is positioned suchthat its channels are biased towards the photodetector's first end 80 a.The second functionalized MCP 82 b is positioned so that its channelsare biased toward the second end 80 b of the photodetector 80 b. A biasmay be applied between the two MCPs to accelerate electrons across thegap between the MCPs. This so-called chevron configuration increases theprobability that electrons leaving the first MCP 82 a will collide withthe walls of the channels within the second MCP 82 b while minimizingion feedback. The depending end of the photodetector is defined bydetecting means 84 underlying (and therefore downstream of) the MCPs 82.The detecting means 84 overlays the depending window 81 b. Suitabledetecting means are a metal film d to collect and integrate the totalcharge emitted from the MCPs 82, a phosphor screen to convert collectedelectrons into an optical pattern that can be recorded on a camera.Alternatively, the detecting means can be a segmented anode such as aseries of discrete metal strips or crossed delay lines.

There is a space between photocathode 83 and the superior MCP 82 a and abias across this space may be applied to accelerate the photoelectronsto increase detection efficiency. Finally, there is a space between theMCPs and the collector of electrons (detector 84) on the back such as aphosphor screen. Again, a bias may be applied between the second MCP 82b and the detector 84 for electron acceleration. Photodetectors can alsobe fabricated using one, two, three, or more MCPs 82 where eachadditional MCP provides an additional gain factor of 1000.

In an embodiment where the two MCPs 82 a and 82 b may be printedtogether with a conducting layer between to provide a precise registrybetween the pores of the two plates.

In use, when light A hits the negatively charged photocathode 83, itejects an electron. An electron ejected from the photocathode 83 thencollides with the interior of one of the channels of the first MCP 82 a.As the MCPs are functionalized as shown in FIG. 7 and described above,the channel of the first MCP 82 a ejects additional electrons uponcollision with the first electron. These electrons then continue towardthe detecting means, colliding into the channel walls of the MCPsadditional times along the way. With each collision between an electronand the walls of an MCP channel, additional electrons are ejected. Inthis way, the MCPs multiply electrons ejected from the photocathode 83.The path and multiplication of the electrons through the MCPs isrepresented by the stippled element labeled element 86.

In the photodetector configuration, each MCP has a bias applied acrossit such that that the superior ends (85, 87) of the MCPs are up to 1.5kV more positive than the depending ends (89, 91). The detector, 84 is50-200 V more positive than the depending end 91 of the second MCP 82 b.Typically, the superior end 87 of the second MCP 82 b is between 0-200 Vmore positive than the depending end 89 of the first plate. In thisconfiguration, the photocathode is biased at approximately −2500 V whichis a few hundred volts more negative than the superior end 85 of thefirst MCP 82 a.

In a neutron detecting configuration, there is no photocathode and thesuperior end 85 of the first MCP 82 a is approximately −2200 V. In thephotodetector or other electron detecting configuration, the superiorend 85 of the first MCP is biased at approximately −2000 V with theanode (detector 84) at approximately 4000 V.

FIG. 9 depicts an exploded view of the complete configuration of thephotodetector 80 shown in simplified cross section in FIG. 8. In FIG. 9,the periphery of the photocathode 83 is surrounded by a continuousconductive border 90 so that the cathode can be charged. Additionally,when fully assembled, the photodetector 80 uses three grid spacers 92,one between the photocathode and the first MCP 82 a, one between the twoMCPs, and one between the second MCP 82 b and a sealing member 94, thatsealing member 94 positioned between the detecting means and the secondMCP 82 b. Also shown in FIG. 9 is the sidewall 96 that contains allcomponents inferior to the photocathode and superior to the detectingmeans when the photodetector is assembled as shown in FIG. 10.

Where the instant MCPs are printed to contain ¹⁰B as discussed above,the design of the detector shown in FIG. 8 is easily modified to detectneutrons instead of photons. Alternatively, the MCPs can be printed tocontain ¹⁵⁷Gd, or any element with a high cross section for neutroncapture. A simplified cross section of such a detector 110 is shown inFIG. 11. In this detector, a third MCP 112 is added superior to the two82 shown in FIG. 8. This third MCP 112 is doped with ¹⁰B as discussedabove and is thinner than the other two (82) MCPs in the detector. In anembodiment, the third MCP 112 is between 10 and 100 μm in thickness. Inthis detector 110, the photocathode 83 is removed such that a window 81is the most superior layer.

In the neutron detector configuration, the ¹⁰B doped MCP 112 willproduce electrons upon incidence of a neutron according to the Equation1 with a Q value of 2.31 MeV, a α kinetic energy of approximately 1.470MeV, and a γ energy of approximately 0.48 MeV:

¹⁰B+n→⁷Li+α+γ  Eq. 1

According to Equation 1, using the detector shown in FIG. 11, a neutronincident on the doped MCP 112 will eject an electron that will then bemultiplied as shown in FIG. 8.

In an embodiment, the detectors described above are assembled entirelythrough 3D printing. The functionalizing elements of the MCPs discussedabove, may be printed simultaneous with the MCP itself as well as theother elements shown in FIGS. 8-11.

In another embodiment, the neutron sensitivity can be imparted bycoating the MCP polymer structure with a coating or film containing ¹⁰B.This coating can be deposited by ALD using ¹⁰B-doped precursor. Othermethods include a solution-phase sol-gel process, CVD, electrodepositionof a conducting ¹⁰B-containing film, electroless deposition, andcombinations thereof.

EXAMPLES

Two MCPs were printed according to the instant method, each having adiameter of 1 cm and a thickness between 0.7 and 0.9 mm. Both sampleswere functionalized with the deposition of a gold electrode via thermalevaporation on the ends of the hexagonal channels as shown in FIG. 7A.The pores in the interior of the hexagonal channels were then sealedusing SIS of Al2O3 as shown in FIG. 7C with 25 SIS Al2O3 cycles at 175°C. The interiors of the channels were then coated with a resistivecoating and secondary emission coating as shown in FIG. 7A. The tunableresistive coating comprising a nanocomposite of Al2O3 and tungsten witha thickness of 50 nm prepared using a W:Al2O3 cycle ratio of 33% at 175°C. was deposited using ALD. Finally, a secondary electron emissivecoating of 10 nm Al2O3 prepared using 75 ALD Al2O3 cycles at 175° C. wasapplied to the resistive layer using ALD. The resulting MCP had aresistance of 2.3×109 Ohms. Gain production was then demonstrated onboth of the functionalized MCPs.

Example 1

The second 3D printed, functionalized MCP was tested in a large vacuumphosphor screen equipped test chamber. In this test, ultraviolet lightwas used to illuminate the 3D printed, functionalized MCP directly whichproduced electrons that were multiplied in the 3D printed MCP.Additional gain was provided by use of a large 8″×8″ MCP purchased fromIncom, Inc. of Charlton, Mass. that was functionalized via the ALDprotocol discussed above used in the phosphor chamber such thatelectrons exiting the 3D printed MCP entered the larger MCP, were gainmultiplied, and then imaged by a subsequent phosphor screen.

In FIG. 12, the circle indicates the location of the MCP in example 1. Amercury vapor lamp was used to produce UV light that liberated electronsfrom the gold electron of the functionalized MCP. A bias voltage of1000V was applied across the 0.9 mm thickness of the MCP to acceleratethe electrons through the MCP. The electrons struck the channels' wallsas they traverse the MCP producing more electrons and hence gainproduction. Electrons exiting the MCP were further multiplied in a largeMCP purchased from Incom, Inc. of Charlton, Mass. that wasfunctionalized via the ALD protocol discussed above directly downstreamfrom the printed MCP with reference to the position of the UV light. Theelectron cloud exiting the large MCP strikes a phosphor screen producingthe image shown. The halo surrounding the MCP image is thought to beproduced by backscattered electrons from the large MCP.

Example 2

In a small vacuum test chamber, a 33 mm diameter MCP was used to produceelectrons that were then multiplied in one of the 3D printed,functionalized MCPs. Evidence of gain production is in the form of animage of the multiplied electron cloud on a phosphor screen as shown inFIG. 12 and a measurement of the relative gain versus bias voltageapplied to the MCP.

FIG. 13 shows the relative gain of a 3D printed MCP versus bias voltageapplied to the MCP in Example 2. The top curve shows the electroncurrent as the 3D printed MCP bias is varied with the standard MCPvoltage held constant at 1400V. The bottom curve shows the current asthe standard MCP voltage is varied with the 3D printed MCP bias voltageheld at 1300V. The top curve shows the gain produced by the 3D printedMCP increasing by a factor of approximately 500 as the voltage increasesfrom 200V to 1400V.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventionwithout departing from its scope. While the dimensions and types ofmaterials described herein are intended to define the parameters of theinvention, they are by no means limiting, but are instead exemplaryembodiments.

In an embodiment, the invention comprises a gain device comprising aplurality of channels having a polygonal shape with four or more sides.The channels having a polygonal shape may comprise hexagonal channels.The polygonal shaped channels may extend transversely through the deviceso as to define a first channel end and a second channel end. The devicemay be at least 1.2 mm thick, and have a diameter that is at least onecm. The device has a latitudinal axis perpendicular to the thickness ofthe device and the channels having a polygonal shape extend at an angleof between 0° and approximately 30° relative to the latitudinal axis ofthe device. In this embodiment, the first and second ends of thechannels may be coated with a conductive layer. The conductive layer maybe made from gold, platinum, palladium, nichrome, copper, andcombinations thereof. The polygonal channels may further comprisegraphene oxide. The channels having a polygonal shape further compriseinterior surfaces, wherein the interior surfaces are coated with a firstresistive coating and a secondary electron emissive coating. Theresistive coating comprises a combination of Al₂O₃ and tungsten, andwherein the secondary electron emissive coating is made from a materialselected from the group consisting of Al₂O₃, MgO, and combinationsthereof. The first coating is between 10 and 1000 nm thick and thesecond coating is between 1-100 nm thick. The device may comprise anopen area ratio of at least 80 percent. The device provides 10⁴ gain.The device may be incorporated into an electron multiplication devicecomprising a source for electrons positioned superior to the device. Theelectron multiplication device may further comprise a second inventedgain device positioned inferior to the first gain device.

In another embodiment, the invention provides a method for producingmicrochannel plates (MCPs) comprising providing a pre-polymer; anddirecting a laser over the pre-polymer in a pre-determined pattern. Thepre-polymer may comprise a mixture of Pentaerythritol triacrylate,9,9-Bis[4-(2Acryloyloxyethoxy)phenyl]fluorene, and2-(o-Phenylphenoxy)ethyl acrylate (<24%). The method may produce an MCPwith a diameter of at least one cm and a thickness of at least 1.2 mm.The invented method may take less than 24 hours to complete. In thisembodiment, the produced MCP comprises a plurality of hexagonalchannels. The method may further comprise coating terminating ends ofthe hexagonal channels with a conductive coating. In the method, thehexagonal channels may further comprise interior walls and the methodfurther comprises depositing a first coating on the interior walls ofthe hexagonal channels and depositing a second coating on top of thefirst coating. The method may further comprise the steps of: depositinga resistive layer on an interior surface of the channels; and depositinga secondary electron emissive coating on the resistive layer. In themethod, the resistive coating comprises a combination of Al₂O₃ andtungsten and the secondary electron emissive coating is made from amaterial selected from the group consisting of Al₂O₃, MgO, andcombinations thereof. The method may further comprise the step ofsealing pores in the interior surfaces of the channels before depositionof a resistive layer. In this embodiment of the invention, the pores inthe interior surfaces of the channels are sealed with Al₂O₃.

Another embodiment of the invention provides a method for efficiently 3Dprinting an object comprising: a) providing a pre-polymer into a sampleholder; b) directing a laser over the pre-polymer in a predeterminedpattern to create a layer of an object having a height, H; c) raisingthe sample holder along a latitudinal axis through the center of thelayer by a distance equal to H; d) directing the laser over thepre-polymer in the predetermined pattern; e) repeating steps a)-d) untilthe object has a first predetermined area and a predetermined secondheight; f) resetting the position of the sample holder; g) moving thesample holder along a latitudinal axis of the layer a predetermineddistance; h) repeating steps a)-g) until the object has a final area; i)resetting the position of the sample holder; j) raising the sampleholder along a latitudinal axis through the center of the layer by adistance of the predetermined second height plus H; and k) repeatingsteps a)-j) until the object has a final area and final height.

Many other embodiments will be apparent to those of skill in the artupon reviewing the above description. The scope of the invention should,therefore, be determined with reference to the appended claims, alongwith the full scope of equivalents to which such claims are entitled. Inthe appended claims, the terms “including” and “in which” are used asthe plain-English equivalents of the terms “comprising” and “wherein.”Moreover, in the following claims, the terms “first,” “second,” and“third,” are used merely as labels, and are not intended to imposenumerical requirements on their objects. Further, the limitations of thefollowing claims are not written in means-plus-function format and arenot intended to be interpreted based on 35 U.S.C. § 112, sixthparagraph, unless and until such claim limitations expressly use thephrase “means for” followed by a statement of function void of furtherstructure.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” “greater than,” “less than,” “more than”and the like include the number recited and refer to ranges which can besubsequently broken down into subranges as discussed above. In the samemanner, all ratios disclosed herein also include all subratios fallingwithin the broader ratio.

One skilled in the art will also readily recognize that where membersare grouped together in a common manner, such as in a Markush group, thepresent invention encompasses not only the entire group listed as awhole, but each member of the group individually and all possiblesubgroups of the main group. Accordingly, for all purposes, the presentinvention encompasses not only the main group, but also the main groupabsent one or more of the group members. The present invention alsoenvisages the explicit exclusion of one or more of any of the groupmembers in the claimed invention.

The embodiment of the invention in which an exclusive property orprivilege is claimed is defined as follows:
 1. A gain device comprisinga plurality of channels having a polygonal shape with four or moresides.
 2. The gain device of claim 1 wherein the channels having apolygonal shape are hexagonal channels.
 3. The gain device of claim 1wherein the channels having a polygonal shape extend transverselythrough the device so as to define a first channel end and a secondchannel end.
 4. The gain device of claim 1 wherein the device is atleast 1.2 mm thick, and wherein the device has a diameter that is atleast one cm.
 5. The gain device of claim 3 wherein the first and secondchannel ends are coated with a conductive layer.
 6. The gain device ofclaim 5 wherein the conductive layer is a conductive material selectedfrom the group consisting of gold, platinum, palladium, nichrome,copper, and combinations thereof.
 7. The gain device of claim 5 whereinthe channels having a polygonal shape have interior surfaces, andwherein the interior surfaces are coated with a first resistive coatingand a secondary electron emissive coating.
 8. The gain device of claim 7wherein the resistive coating comprises a combination of Al₂O₃ andtungsten, and wherein the secondary electron emissive coating is madefrom a material selected from the group consisting of Al₂O₃, MgO, andcombinations thereof.
 9. The gain device of claim 7 wherein the firstcoating is between 10 and 1000 nm thick and the second coating isbetween 1-100 nm thick.
 10. The gain device of claim 3 wherein thedevice has an open area ratio of at least 80 percent.
 11. The gaindevice of claim 4 wherein the device provides 10⁴ gain.